The present invention pertains to the field of measuring wavelengths in a spectrum of light. More particularly, the present invention pertains to a method and corresponding apparatus for correcting systematic error in a wavelength measuring device.
The use of Fiber Bragg grating (FBG) devices in telecommunications sensing has driven the requirements for very accurate measurements to be made on the center wavelengths of these devices, often down to 10xe2x88x9212 meters. Various instruments today are able to provide highly accurate readings of FBGs, especially when the FBGs function in a reflective mode. However, currently, many of these instruments cannot provide accuracies down to the 10xe2x88x9212 meters often required. What stands in the way are various errors associated with the instruments, some because of how the instruments are affected by the environmental conditions in which the instrument is used, including the ambient temperature, or because of aging of the instruments. Some of these errors are systematic, so that the instrument makes the same error in reading any FBG.
For example, as shown in FIG. 1, one method often used to measure FBG reflected wavelengths utilizes a scanning Michelson interferometer as a component of a wavelength measuring device. Such a method measures interferometric fringes that vary in frequency depending on the wavelength of light entering the interferometer, allowing a determination of the wavelength of the light, but also depending on the rate of the interferometric scan; the variation caused by the changes in the scan rate are a source of systematic error in the determination of the wavelength of the light. A Fourier transform of the output of the interferometer, i.e. the fringe pattern produced by the interferometer, is then made to characterize (in terms of frequency or wavelength components) the light entering the interferometer. In other words, by determining a Fourier transform of the output of the interferometer, the wavelength of light reflected from a FBG can be determined.
As shown in FIG. 1, light reflected from a reference laser, and so having a peak at an approximately known wavelength (the wavelength varies somewhat depending on ambient conditions such as temperature), is sometimes used to subtract the systematic error caused by changes in the scan rate of the interferometer, as well as the systematic error caused by other factors, including ambient temperature.
Using a reference laser can provide an accurate measurement of the wavelengths of light reflected from sensing FBGs by an interferometer, or more generally a wavelength measuring device, but only if the wavelength of light from the reference laser is known. A change by an unknown amount in the wavelength of the light from a reference laser (because of a change in temperature, for example) will produce an error that will be systematic, i.e. that will occur in the wavelength determination for each peak in a scanned spectrum, but which cannot be corrected, because the error is unknown. To correct systematic error, the magnitude of the systematic error must be determined. Even a very small change in temperature of a reference laser, as little as 0.007xc2x0 C., can cause an unacceptably large change in the wavelength of light from the reference laser.
What is needed is a way of compensating for systematic error in a wavelength measuring device, such as a device that includes a Michaelson interferometer, the compensating based on taking into account what the wavelength measuring device measures to be the wavelength of light provided by a reference device, not necessarily a laser, and doing so in a way where the wavelength of the reference light is in fact known with great precision, and in particular is known to a precision that distinguishes changes in the reference wavelength caused by changes in the temperature of the reference device by as little as 0.007xc2x0 C.
Accordingly, the present invention provides an apparatus for compensating for systematic error in wavelength measurements provided by a wavelength measuring device used for measuring the wavelengths of peaks in a spectrum of light, including light reflected from a reference fiber Bragg grating (FBG), the apparatus including a temperature sensing and measuring circuit and a dynamic compensator. The temperature sensing and measuring circuit is for measuring the temperature to which the reference FBG is exposed. It is responsive to the temperature, and provides a signal indicative of the temperature. The dynamic compensator is responsive to the wavelength measurement signals from the wavelength measuring device, responsive to the signal indicative of the temperature of the reference FBG, and further responsive to a signal indicating a correlation of the true wavelength of light reflected from the reference FBG and the temperature of the reference FBG. It provides dynamic compensator signals indicating information about compensated wavelength measurements.
In one aspect of the invention, the information about compensated wavelength measurements is based on the difference between the true wavelength of light reflected from the reference FBG for the measured temperature of the reference FBG, and the measured wavelength of light reflected from the reference FBG.
In another aspect of the invention, the temperature sensing and measuring circuit includes: a switched sensing circuit having a common set of electrical components and two different electrical components, responsive to the temperature to which the reference FBG is exposed, and further responsive to a detection circuit switch control signal, for providing switched sensing circuit signals containing information about an operating characteristic of the switched sensing circuit using the common set of electrical components switched respectively through each of the two different electrical components; and a detection circuit, for providing the detection circuit switch control signal, and responsive to the switched sensing circuit signals, for providing a detection circuit signal containing information about the temperature to which the reference FBG is exposed. In some applications of this aspect of the invention, the common set of electrical components includes a capacitor connected in series with a multi-pole switch, and the operating characteristic is a time to charge or discharge the capacitor.
From another perspective, the present invention includes: a wavelength measuring device, responsive to a light signal having a spectrum including a peak at a wavelength of light reflected from a reference fiber Bragg grating (FBG) exposed to a temperature, for providing a signal indicating a measurement of the wavelength of the peak; and a compensating circuit, responsive to the signal indicating the wavelength measurement, and responsive to the temperature to which the reference FBG is exposed, for providing signals indicating values for other wavelength measurements compensated for systematic error made by the wavelength measuring device.
The method of the invention comprises the steps of: enclosing the reference FBG so as to see substantially a uniform temperature; measuring the temperature; determining the true wavelength of a peak of light reflected by the reference FBG based on the measured temperature and a pre-determined correlation between temperature of the reference FBG and a wavelength of a peak of light reflected by the reference FBG; acquiring the measured wavelength of the peak of light reflected by the reference FBG; and determining a correction to be used to compensate the wavelength measuring device for systematic error based on the difference between the measured wavelength of the peak of light reflected from the reference FBG and the true wavelength of the peak of light reflected from the reference FBG.
In a particular aspect of the method of the invention, the temperature of the reference FBG is measured using a thermistor disposed so as to sense substantially the same temperature as the reference FBG. In addition, a fixed resistor is used, one substantially insensitive to temperature, and the temperature of the reference FBG is measured based on operating characteristics of a circuit involving the thermistor and, in turn, the reference resistor by switching into the circuit the thermistor and, in turn, the reference resistor, but keeping substantially all other components in the circuit exactly the same.